The present invention relates to a flowmeter for measuring and displaying a flow rate of fluid, such as a gas.
A flowmeter utilizing a hot-wire current meter has been known, of which application includes a gas meter. The hot-wire current meter determines flow velocity through the use of the fact that the cooling rate of the hot wire installed in a pipe is a function of the flow velocity of fluid flowing through the pipe. The flowmeter with the hot-wire current meter calculates a flow rate to display from the flow velocity.
FIGS. 1, 2 and 3 show the structure of an example of a conventional flowmeter using a hot-wire current meter. This flowmeter is equipped with a flow velocity sensor 1002 installed, for example, in the center of a pipe 1001. The flow velocity sensor 1002 has a hot wire 1002A which is connected to a DC power source 1003B via a resistor 1003A. Both ends of the resistor 1003A are connected to a computing circuit 1003C which calculates flow rates. A display 1003D which displays a flow rate is connected to the computing circuit 1003C.
If the supply voltage of the DC power source is V.sub.0, the resistance of the hot wire 1002A is r.sub.0, the resistance of the resistor 1003A is R.sub.0, and the electric current flowing through the hot wire 1002A and resistor 1003A is i.sub.0, then the relationship in equation (1) holds: EQU i.sub.0 =V.sub.0 /(R.sub.0 +r.sub.0) (1)
When r.sub.0 &gt;&gt;R.sub.0, equation (1) is approximated by: EQU i.sub.0 =V.sub.0 /r.sub.0 ( 2)
The resistance r.sub.0 of the hot wire 1002A changes due to flow velocity. Therefore, when the voltage V.sub.0 is constant, the current i.sub.0 also changes due to the flow velocity according to equation (2). Consequently, the flow rate Q.sub.0 corresponding to the flow velocity is a function of the electric current i.sub.0, which is expressed by: EQU Q.sub.0 =K.sub.0 .times.(i.sub.0 -i.sub.00) (3)
In equation (3), K.sub.0 is a coefficient according to the pipe and the like, and i.sub.00 is the electric current flowing through the hot wire 1002A and resistor 1003A when the flow rate Q.sub.0 equals zero.
The electric current i.sub.0 in equation (3) is determined by i.sub.0 =v.sub.0 /R.sub.0 when the voltage across the resistor 1003A is v.sub.0. The computing circuit 1003C, shown in FIG. 3, uses equation (3) to calculate the flow rate Q.sub.0. Then, the calculated flow rate Q.sub.0 is displayed by the display 1003D.
However, flow velocity in the pipe 1001 has a flow velocity distribution varied in the same cross section, due to varied shapes, bends and branches in the pipe, and the magnitude of the flow rate. Changes in flow velocity are proportional to changes in a flow rate in the pipe 1001 within a narrow range of measurement locations. Accordingly, in a conventional flowmeter wherein the flow velocity sensor 1002 is located at one position in the pipe 1001, only flow velocity at that location is determined in the pipe 1001. That is, a problem in the conventional flowmeter is that flow rates cannot be accurately determined within a wide measurement range of flow rates.
The present invention is designed in view of the problems described above relating to conventional devices. It is the objective of the invention to provide a flowmeter with a simple structure which accurately measures flow rates, regardless of flow velocity distributions in a pipe.